1. Field of the Invention
The invention relates to method and apparatus for sensing the presence of a moving object and for generating outputs indicative of the path of travel of moving objects and more particularly to method and apparatus for sensing the presence and/or relative position of a rotating fan blade or helicopter blade.
2. Background Art
It is well known that vibration resulting from rotating objects are often detrimental. Both military and commercial rotorcraft operators wage a continual battle against vibration, both to extend the life of the rotorcraft and to enhance the comfort of the crew and passengers. The rotor itself is a principal contributor to vibration. However, rotor vibration can be reduced through proper balancing of the rotor blades. Various devices have been developed and sold over the last several decades to provide the information required to accurately and efficiently balance a rotor. The more accurate and efficient rotor track and balance systems incorporate a blade tracking device to measure blade height and lead/lag between the respective blades of the rotor. Measurements of these blade parameters may be used in a balancing algorithm to generate indicia representing recommended changes in blade adjustments in order to minimize vibration. In addition, blade tracking information is required in an over-all rotorcraft tuning algorithm and for the successful completion of various required maintenance actions.
In order to reduce damage to the engine, transmission, and airframe caused by the vibration of helicopter blades, it is important that all blades of a rotor travel in the same plane and that all blades rotate with a fixed angular separation. To detect that the several blades of a helicopter rotor or the like travel in the same plane, it is a common practice to measure the distant of each of the blades from a fixed point on the craft when the blades are in predefined position relative to the fixed point. Prior art blade tracking systems primarily use strobe lights and optical sensors which measure light reflected from the rotating blades to determine the relative position of the moving blades. Some blade tracking systems have used electrostatic sensing probes or radiation, whereby a lens focuses a beam of radiation in a plane and the sensing probes detect the presence of an object as it passes through the radiation beam. Other prior art systems employ an oscillator connected to a large capacitive element positioned adjacent the plane of rotation of the blades of a rotor. In such a system, blade tracking measurements are derived from frequency modulations, resulting from a change in capacitance, and displayed on an oscilloscope.
A general problem with tracking devices known from the prior art is the lack of accuracy of the generated output signals. Prior art tracking devices using electrostatic probes are prone to error due to undefined static sources and electrostatic changes in the atmosphere which may influence accuracy of any static electricity detectors. Most earlier blade tracking systems, including those systems relying on change in capacitance, typically measure the position of the several blades of a propeller while the craft is on the ground. However, it is highly desirable that improper tracking be detected in flight, since blade dynamics vary based on engine speed and load. Commonly used tracking devices which rely on reflected optical signals are generally inaccurate because of interference due to ambient light and cannot be used at night unless the blades are artificially illuminated. In addition, reflective tape is commonly required on the blades. Furthermore, optical tracking systems tend to be sensitive to color. On helicopter rotors, color may vary from blade to blade and is often white or shades of gray. Optical sensors have been known to be incorrectly triggered by rotor shadows and to be sensitive to varying ultraviolet light levels across the zenith.
Known optical devices detect the passage of rotor blades through their field of view and generate pulse edges as each rotor blade enters and leave the optical sensing region. The path of the rotating blade is detected by using two such optical devices, such that their corresponding optical sensors are separated by a known angle. Precise time measurements of the pulse edges detected at the two optical sensing regions, coupled with rotor speed and installation parameters, allow blade height to be calculated. Synchronized timer circuits are used to identify the time that each blade enters or leaves the optical sensing region of each sensor. A once-per-revolution timing pulse is simultaneously measured to provide both rotational phase and rate information. Specifically, what is measured is the time at which the rotor blade enters the field of view and leaves the field of view of each of the two sensors. Based on this information and using well-known geometric equations, the track height of the rotor blade can be readily determined in a well-known fashion. Furthermore, the actual blade angular velocity can be calculated as well. The accuracy of blade velocity calculations is determined, in large measure, by the ability to accurately identify both leading and trailing blade edges of the blades as they pass through a measurement space. One of the problems with prior art optical systems is the inability of the track sensors to properly respond to light level changes as the blades enter and leave their fields of view. This problem is often aggravated by differences in paint and/or paint erosion on the several blades, which further tends to affect the accuracy of the blade height calculations.
Radio frequency or microwave devices such as the well-known Doppler or pulsed radar systems have commonly be(n used as locating and tracking devices. However, known radar devices such as Doppler radar depends on change in frequency and uses complex circuitry to measure changes in transmit frequency. Pulsed radar typically can be used effectively only when the distance between the source and the target is greater than 100 meters and is therefor not suited for use as a helicopter blade tracking device, where the tracking device may have to be mounted approximately one meter from the track of the rotor blades.
These and other problems of the prior art are overcome in accordance with the present invention by means of tracking apparatus comprising a radiating antenna having a predefined antenna impedance and detection circuitry for detecting deviation in antenna impedance resulting from movement of an object within the field of radiation of the antenna. Advantageously, the antenna may be positioned in close proximity to the projected path of a moving object, e.g. at a distance of on the order of one meter or less.
In accordance with one aspect of the invention, the tracking apparatus is used in a system for tracking the several blades of a helicopter rotor and the antenna is directed toward the path of travel of the rotor blades. A signal source connected to the antenna transmits radio frequency signals to the antenna, causing the antenna to radiate an electromagnetic field in the path of the helicopter rotor blades. Signal detection circuitry, connected to the antenna, provides an output signal when an a change in antenna impedance occurs due to the entry of a rotor blade in the field of the antenna. A computation of distance of each blade from the antenna is derived in a standard fashion to provide a measure of tracking of the rotor blades.
In a specific embodiment of the invention, a radio frequency (RF) generator connected to the antenna provides a continuous wave unmodulated signal. As a moving blade enters the beam, it causes a change in the antenna field impedance, resulting in a modulation of the transmitted signal. The modulated signal is received by the antenna and is detected and amplified by circuitry connected to the antenna.
Advantageously, the tracking arrangement of the present invention comprises a compact antenna structure consisting of printed circuit boards which are readily and unobtrusively mounted.
In a particular embodiment of the invention, a tracking arrangement comprises a pair of spaced apart antennas, each connected to a circuit arrangement including a signal source and circuitry of the present invention for detection of change in impedance, whereby distance of a moving object from a predetermined location may be computed from signals indicative of time of detection at the spaced apart antennas.
In accordance with one aspect of the present invention, a signal source transmits an RF signal to an antenna, directed toward the path of an object to be detected, causing the antenna to radiate an electromagnetic field in the direction of the path of the object. The antenna preferably has a well predefined antenna impedance resulting in a well defined return loss. As the object passes through the electromagnetic field of the antenna, the impedance of the antenna changes, resulting in a change in the return loss. Effectively, movement of the object through the antenna field causes an amplitude-modulation of the carrier signal. In addition to causing a change in the amplitude of the return signal, the moving object also causes a change in the phase of the return signal with respect to the transmitted signal, resulting in a phase modulation.
Advantageously, the blade tracking device in accordance with the invention acts as a motion detector which is sensitive to change in distance between the antenna and the moving objects.
Advantageously, the detector device of the present invention is not sensitive to light and can be used in bright sunlight as well as in darkness. Neither do clouds, nor fog, nor the color of the items being tracked have any effect on the measurements obtained by the tracking device of this invention. Furthermore, the antenna detector device of the present invention is small and may be housed in a standard radome capable of withstanding normal environmental changes such as moisture, sand, dust, and dirt as well as vibration and shock or exposure to manmade corrosives such as fuels and lubricants.